Positive Parallax and Negative Parallax
When we watch 3D there are several factors at play to make it work well. One is to do with the parallax of the image. If you imagine the X axis (horizontal axis) of a 3D image as divided into 3 parts.....foreground, middle and background, then the middle would be the stereo window (or the actual screen) while the background and foreground would appear behind and in front respectively. These are called Positive Parallax (background) and Negative Parallax (foreground). In the stereo window (or middle) the stereo pairs would appear to be in alignment (or almost in alignment) so that the objects would appear to be in focus. Alternatively, those items in the background and foreground would appear to have a double image where the disparity (separation) of the stereo pairs is creating either the 3D depth or 3D pop out effect. The wider the disparity, the better the depth and pop-out effect although there will be a tipping point. If the disparity is too wide, then the 3D effect may be lost altogether. Too small a disparity creates a weak 3D effect with little depth or pop out. Unfortunately, a lot of new movies being released in 3D have a narrow disparity and are creating an insipid 3D that audiences are tiring of. When 3D is done well, it should always wow the audience. Notice how in the above anaglyph image, the sides have reversed in both the positive and negative parallax. In the Positive the green filter is left and red right while vice versa for the negative.
If you compare the disparity in the above image you will see that the parallax between both pairs is quite large. This has the affect of creating excellent 3D where there is both great depth and pop out.
For more information and illustrations about Positive and Negative Parallax go Here
The above images are taken from the LG 3D Demo Global which can be viewed Here